The present invention relates to a system and method for achieving extended low frequency response and output sound pressure level capability at low frequencies.
In loudspeaker systems, especially high quality audio systems that are intended to produce the full range of audible signals, particularly those at low frequencies, a major design challenge lies in achieving adequate low-frequency extension, both in terms of low frequency response and maximum achievable sound pressure levels (SPL) at low frequencies. This challenge is further increased when this performance must be achieved in a small enclosure, or with small loudspeaker drivers, or both.
One of the major challenges in loudspeaker system design, in terms of low-frequency performance, is to achieve a frequency response that extends to low frequencies in or below the 30-50 Hz range. A more difficult challenge is to achieve high output sound pressure levels (SPL) at these same low frequencies, owing to the need to move large amounts of air in order to achieve high sound pressure levels. Because the maximum cone excursion of the driver determines the amount of air that can be moved (in combination with the driver's effective cone area), this limitation is referred to as Excursion-Limited SPL, or ELSPL. The ELSPL of a driver is a function of frequency, and typically decreases at lower frequencies because at low frequencies a correspondingly larger amount of air must be moved to achieve a given SPL.
Most conventional loudspeaker system designs representative of the prior art fall into one of two broad categories. Sealed systems, often called closed-box systems, or acoustic suspension systems, provide a second-order high-pass frequency response. They suffer from higher low-frequency −3 dB cutoff frequencies (f3) and low ELSPL. The low frequency cutoff frequency of a sealed system can be reduced, but at the expense of a much larger box. Alternatively, the f3 of such a system may be reduced by employing a heavier cone, which reduces the resonant frequency of the system. Use of this latter technique usually results in much reduced electro-acoustic efficiency. In either case, however, low-frequency ELSPL is not increased.
Ported systems, also known as vented systems or bass reflex systems, add a port to the box in which the driver is mounted, forming a Helmholtz resonator. When properly designed, the box-port Helmholtz resonance produces a lower f3 and also produces a higher ELSPL at low frequencies. In such systems, the box-port Helmholtz resonant frequency is referred to as fb. These systems provide a fourth-order high pass frequency response. As frequency is reduced from higher frequencies down to f3 and then to frequencies below f3, the frequency response begins to fall off very sharply, at a rate approaching 24 dB/octave. The steep rolloff typically begins at frequencies below the box tuning frequency fb. The steep low frequency rolloff tends to cause group delay distortion and poor transient response. Although ported systems provide increased ELSPL at frequencies above f3, the ELSPL of ported systems falls off severely at frequencies below fb, providing virtually no useful output at such frequencies. Ported systems actually produce LESS ELSPL than that of a comparable sealed system at frequencies below fb of the ported system.
One commercial example of a low-frequency sealed system designed for extended low-frequency performance is a subwoofer implemented by Carver Corp. (U.S. Pat. No. 6,566,960). It is essentially a brute-force sealed system that employs a special driver with very large cone mass and very large cone excursion. The design results in very low efficiency and requires extremely high drive power. The very high cone mass also compromises transient response.
A small number of sealed systems employ equalization in order to achieve an extended low frequency response with a reduced f3. This approach does not suffer from the approaches mentioned above wherein larger cabinets or reduced electrical efficiency is required. Such equalization is most often done with an active filter placed in the signal path prior to the power amplifier that drives the loudspeaker. These equalizers typically provide a biquadratic filter function that includes a pair of zeros and a pair of poles. The pair of zeros is typically placed at or near the same frequency as the pair of poles produced by the unequalized sealed system. The pair of biquadratic poles is placed at a lower frequency corresponding to the desired equalized f3 of the system. Such an equalizer is also well known to those familiar with the prior art as a Linkwitz Transform.
This technique, referred to here as an Equalized Sealed System (ESS), is very effective at improving the frequency response of the sealed system loudspeaker. However, it also does nothing to improve or increase the low-frequency ELSPL. Therefore, in order to be practical, and to have an ELSPL commensurate with the extended low frequency response afforded by the ESS technique, such systems typically must employ a large driver with a very large excursion capability. Such systems may typically employ equalization to move the system f3 down by about one octave. This corresponds roughly to 12 dB of equalization, which in turn corresponds to an increased power of 16 times at the f3 of the equalized system. This is a direct consequence of the greatly reduced efficiency of a sealed system at frequencies below its unequalized f3. As a result, large power amplifiers are often required for use with such systems.
The Bag End ELF system (U.S. Pat. No. 4,481,662) is a commercial example of an equalized sealed system. This system comprises essentially a double integrator equalizer placed in the input signal path of a sealed system. This is an alternative to the above-mentioned Linkwitz Transform, and has all of the same shortcomings. In particular, this approach does nothing to improve ELSPL. Yet another equalized sealed system is described in Russell U.S. Pat. No. 3,715,501.
Ported systems can in principle be equalized, but in practice they virtually never are equalized. This is partly due to the greater difficulty of accurately equalizing a fourth-order system. More importantly, however, is the fact that it makes little sense to equalize a conventional ported system to achieve a lower f3, since the fb of a conventional ported system usually lies near the box tuning frequency, and the ELSPL drops off severely at frequencies below fb. For these reasons, it has heretofore usually been impractical to equalize ported systems.
One example of combining an “equalizer” with a ported system is claimed by Bose Corp. (U.S. Pat. No. 4,154,979). This is merely a variant of the well-known 6th order Chebeychev vented alignment originally described by Theile. This approach provides a small amount of bass extension at the expense of a much worse transient response. The active filter in this approach is essentially a second-order high-pass filter, unlike the low-pass equalizer characteristic of the present invention. This approach also does little for low-frequency SPL capability.
Known approaches and arrangements for achieving extended low-frequency performance are thus sub-optimal in one or more of the performance metrics that include f3, ELSPL, efficiency, box size and transient response. All of the above-mentioned approaches, techniques and inventions fail to realize the combined benefits of the present invention.